Tank storage of hazardous materials, for example petroleum products such as gasoline and oil, compressed gas, low level radioactive waste materials, and the like, has until recently involved the use of underground tanks, usually of steel. Such an arrangement was considered the most effective means for the elimination of potential fire hazards, exposure of persons to noxious or otherwise detrimental fumes, spillage and other equally undesirable occurrences.
However, the burying of storage tanks has, of late, been found to be far from the most practical or desirable solution to the problems of hazardous material storage.
The tendency for underground tanks to deteriorate, as by corrosion, and leak over a period of time, when considered with the general inaccessibility of such tanks, requires corrective action which can be excessively burdensome and expensive for the tank owner. This, when coupled with the increased awareness of the fragility of and necessity to protect the environment, has resulted in federal intervention, for example the Resource Conservation and Recovery Act of 1984 which sets forth strict regulations for the monitoring and testing of underground tanks.
In light of the significant problems associated with buried storage tanks, increase consideration has been given to aboveground tanks which are subjected to less stringent restrictions, and which are in turn principally concerned with compliance to state and local fire and safety codes which can be readily met by enclosing the storage tank itself, whether of steel or fiberglass, within a vault. As a preferred example, the vault should be formed with concrete walls at least six inches thick and providing a firewall structure in conjunction with the tank rated at a minimum of two hours at 2000.degree. F.
One proposal to accommodate the requirements of an aboveground storage system is set forth in U.S. Pat. No. 5,033,638, issued Jul. 23, 1991 to Cruver et al. This patent is of particular interest in setting forth the problems associated with in ground tanks, and the desirability of avoiding such problems by providing for aboveground storage.
The solution proposed by Cruver et al includes surrounding the storage tank itself with spaced pairs of vertically continuous semi-circular ribs having spaced upper and lower ends and intimately engaging the circumference of the received tank for the full extent of the ribs. The rib-supported tank is in turn enclosed within a concrete casing formed of upper and lower semi cylindrical shells with integral end walls, the lower shell including two sets of stabilizing legs.
The Cruver et al vault, formed in two precast semi cylindrical sections, includes distinct advantages over vaults formed in situ, including being able to form the sections at a separate manufacturing facility for subsequent transportation to the point of use. While such an arrangement is feasible with regard to smaller standard size tanks, problems with regard to both transportation and handling could be substantial if the Cruver sections were to be made to accommodate tanks of any substantial diameter and length. In this regard, it is to be noted that petroleum storage tanks are currently factory produced in a wide range of sizes, ranging from 500 gallons to 50,000 gallons. Similarly, diameters vary from four feet to twelve feet and lengths can run from 5 feet to 50 feet. The primary limit on a given tank's size is the highway restrictions on truckload freight, including height, width, length and weight. Weight becomes a particularly significant factor when the vault or vault sections, normally formed of 6" thick concrete, are to be transported.